The present invention relates to an A.C. motor control system and, more particularly, a control system to permit a variable speed operation of an A.C. motor by converting D.C. power into an A.C. voltage with variable frequency through a thyristor inverter.
It is common to provide individual commutating means for each of the thyristors constituting an inverter for operating an A.C. motor at a variable running speed. Since such a system having individual commutating means for the respective thyristors allows the thyristors to be individually turned on or off at a desired time, motor control may be effectively done with a large degree of freedom. Further, the A.C. motor may be operated with a voltage waveform desirable for itself. However, this system is not suitable for the variable speed operation for a small A.C. motor, such as a small asynchronous motor, whose operation efficiency or torque pulsation is not a serious problem in the point that the construction of the system is excessively large.
In this respect, another system to overcome above disadvantages has been proposed in which a single commutating means is provided for the respective thyristors of the inverter to simplify the structure of the system.
Japanese Patent Publication No. 24447/67, for example, discloses such a system in which a commutating means composed of a reactor and a capacitor is provided. In the system, when turning off the thyristors which are conductive then, an oscillating current generating circuit including the reactor and the capacitor is formed to generate a high frequency A.C. signal to invert, during a relatively short time, the current flowing through the conducting thyristors, to turn them off.
Therefore, the reactor is essential to this system and thus the system structure thereof is also large.